Methods for operating a fuel cell of a hybrid vehicle, wherein the hybrid vehicle comprises an electric drive having a power accumulator and also the fuel cell, as an additional power source, are known per se.
Such hybrid vehicles use at least one electric motor as a drive and a fuel cell as an additional power source to extend the range of the vehicle. The fuel cell therefore represents a so-called “range extender” of the hybrid vehicle. The drive of such a vehicle can be performed, for example, primarily or exclusively via the electric drive. In any case, the fuel cell can be used to charge the power accumulator of the electric drive, to thus increase the range of the electric travel. The vehicle can possibly also be driven directly via the fuel cell, depending on its construction.
In comparison to hybrid vehicles having an internal combustion engine, serious differences occur in vehicles having a fuel cell. For instance, the optimization of the efficiency of the auxiliary drive is not so relevant, because the efficiency of a fuel cell is nearly constant on a broad power range. Also, the optimization to the dynamic response of the auxiliary drive is more important, because, to lengthen the service life, a fuel cell cannot be operated dynamically comparably to an internal combustion engine.
Frequently turning a fuel cell on and off can additionally shorten the service life. Therefore, an operating strategy of a hybrid vehicle having internal combustion engine cannot simply be assumed for vehicles having fuel cells. Frequent turning on and off, for example, as in start-stop traffic or when recuperating energy and also high dynamic response, corresponding to the driver command, would have negative effects on the service life.
The operating strategy of the fuel cell during travel should therefore have as little dynamic response as possible and few on/off cycles, to take into consideration the characteristics of the fuel cell.
In addition, there is the power provision by the fuel cell in the form of a range extender (also referred to as REX hereafter). In the case of a range extender, the power of the REX is less in principle than that of the drive engines. If the battery, the power accumulator of the electric drive, is empty, it is not possible to travel with full power, but rather only with the power of the REX. Because this is accompanied by losses or limitations in the drivability, this state should be avoided. Therefore, before phases having high power demand, the power has to be collected/buffered by the REX in the battery in a timely manner, to thus avoid the power deficit of the REX and/or not let it become perceptible to the driver.
In addition, fuel cells always have a minimum power not equal to zero in operation in principle. That is to say, an “idle” as in the case of internal combustion engines is not possible in fuel cells. In particular in city regions, this is problematic if the average power demand of the vehicle is less than the minimum power of the fuel cell. In this case, this would result in continuous charging of the battery.